4. To make Frosting, in large bowl, beat softened butter and powdered sugar with electric mixer on low speed until incorporated; beat on high speed 3 minutes. Add vanilla and salt; beat 1 minute longer. Add blue food color 1 drop at a time, beating until a light blue color is achieved.

5. Fill and frost cooled cake. Refrigerate frosted cake 1 hour or until frosting is dry to the touch.